Conventional closed loop fuel control systems are responsive to the sensed level of oxygen in the combustion gases produced by the engine as measured by an exhaust gas oxygen sensor. The controller responds to a lean condition by progressively increasing the fuel delivery rate until a rich condition is detected, then decreases the fuel delivery rate until a lean indication appears, and so on. In this way, the closed loop controller can maintain the air/fuel mixture at or near stoichiometry to minimize undesirable combustion products.
The control system achieves the desired air/fuel ratio more rapidly when the open-loop portion of the system is able to accurately predict the needed air/fuel ratio based on sensed engine operating conditions (engine speed, load) etc. Adaptive control systems better achieve this accuracy while accommodating production variations in airflow measuring and fuel metering devices used in different engines, by storing parameters developed by the closed loop controller for future use. During subsequent engine operations, the previously generated and stored parameters are retrieved to enable the closed loop controller to more rapidly and more accurately achieve desired air/fuel ratios.
Highly effective fuel control strategies have been developed to help reduce engine emissions while maintaining acceptable engine performance. For example, during idling conditions when the engine is not required to do useful work other than driving accessory loads, fuel consumption and undesirable emissions may be minimized by adjusting the air/fuel ratio to an unusually lean mixture which still provides steady and reliable cylinder firing. The extent to which such strategies can be aggressively pursued with production vehicles is limited, however, by variations from vehicle to vehicle. Air/fuel calibration which would work well for the average vehicle is not chosen if it could impair driveability when used with some vehicles within the expected range of manufacturing tolerances.